In an article entitled "A piezoelectric micropump based on micromachining of silicon" which appeared in "Sensors and Actuators" No. 15 (1988), pages 153 to 167 , H. van Lintel et al describe two embodiments of a micropump each comprising a stack of three wafers, i.e. a wafer of machined silicon arranged between two wafers of glass.
The silicon wafer defines a pumping chamber with one of the glass wafers, it being possible for the part coinciding with this chamber to be deformed by a driving element, in the present case a piezoelectric crystal. The latter comprises electrodes which, when connected to a source of alternative potential, cause deformation of the crystal and thus of the glass wafer, the latter in turn causing variation in the volume of the pump chamber.
The pump chamber is connected on one side and on the other side respectively to check valves machined from silicon the seat of which is composed of the other glass wafer.
Study of the mode of operation of the pump according to the first embodiment (FIG. 1a) described in the above mentioned article shows that the fluid output delivered by this pump depends greatly on the outlet pressure over the entire operating range. It was found that this relationship between output and pressure is virtually linear, the output falling as pressure rises.
In other words, a pump of this type cannot be used for the above mentioned medical applications in which, in contrast, the output from the pump outlet must be independent of the pressure, at least in the normal operating range of the pump.
For this reason the authors propose in this same article (second embodiment shown in FIG. 1b) adding a regulating valve to the above described assembly disposed between the second valve downstream from the pumping chamber and the pump outlet. This valve isolates the pump from the outlet when it is closed.
Moreover, since the regulating valve has a certain degree of mechanical pretension towards closure, the outlet pressure is only able to open the valve after reaching a certain value. As a result, a form of independence of output in relation to outlet pressure develops in the useful operating range of the pump, i.e. when the regulating valve is not kept open by the outlet pressure.
Thus, although this second design gives a favourable output pressure diagram, it should nevertheless be noted that a pump manufactured in this manner still has disadvantages.
The regulating valve increases the pump size since it must be situated within the silicon wafer and thus takes up additional surface area. It also increases the cost price of the pump.
It should also be noted that the regulating valve increases the complexity of the pump and thus the risk of incorrect functioning or manufacturing error.